Magnetoresistive Random Access Memory (MRAM) has a read function based on a tunneling magnetoresistive (TMR) effect in a MTJ stack wherein a tunnel barrier is formed between a free layer and a reference layer. The free layer serves as a sensing layer by switching the direction of its magnetic moment in response to external fields (media field) while the reference layer has a fixed magnetic moment. The electrical resistance through the tunnel barrier (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby provides an electrical signal that is representative of the magnetic state in the free layer. In MRAM, the MTJ is formed between a top conductor (electrode) and bottom conductor. When a current is passed through the MTJ, a lower resistance is detected when the magnetization directions of the free and reference layers are parallel (“0” memory state), and a higher resistance is noted when they are anti-parallel (“1” memory state). The TMR ratio is dR/R where R is the minimum resistance of the MTJ, and dR is the difference between the lower and higher resistance values. The tunnel barrier is typically about 10 Angstroms thick so that a current through the tunnel barrier can be established by a quantum mechanical tunneling of conduction electrons.
Another version of MRAM that relies on a TMR effect, and is referred to as a spintronic device that involves spin polarized current, is called spin-transfer torque (STT) MRAM and is described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996). J-G. Zhu et al. has described another spintronic device called a spin transfer oscillator (STO) in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry.
MTJ elements wherein one or both of the free layer and reference layer have perpendicular magnetic anisotropy (PMA) are preferred over their counterparts that employ in-plane anisotropy because the former has an advantage in a lower writing current for the same thermal stability, and better scalability for higher packing density which is one of the key challenges for future MRAM applications. In MTJs with PMA, the free layer has two preferred magnetization orientations that are perpendicular to the physical plane of the layer. Without external influence, the magnetization or magnetic moment of the free layer will align to one of the preferred two directions, representing information “1” or “0” in the binary system. For memory applications, the free layer magnetization direction is expected to be maintained during a read operation and idle, but change to the opposite direction during a write operation if the new information to store differs from its current memory state. The ability to maintain free layer magnetization direction during an idle period is called data retention or thermal stability. The level of stability required is usually related to the memory application. A typical non-volatile memory device may require thermal stability at 125° C. for about 10 years.
Moreover, for MRAM devices that are often embedded in Complementary Silicon Oxide Semiconductor (CMOS) chips, the MTJ must be able to withstand high temperature processing conditions up to about 400° C. that are commonly applied during the deposition of low-k dielectric films for transistors in CMOS structures. In most cases, this temperature exceeds the optimum temperature for best magnetic performance in the MTJ or MRAM. MTJs are usually annealed in the 300-330° C. range to obtain the desired magnetic properties.
As a result of 400° C. processing, free layer PMA is typically reduced and thermal stability is less compared with a condition where the MTJ is annealed only to 330° C., for example. Free layer coercivity is also less after high temperature processing to around 400° C. than after 300-330° C. annealing. However, it is an important requirement to maintain coercivity after high temperature processing.
Thus, there is a significant challenge to maintain PMA and enhance thermal stability of reference and free layers to improve the performance of MTJs at elevated temperatures typical of back end of line (BEOL) semiconductor processes. Current MTJ structures fail to satisfy the performance requirements for advanced embedded MRAM devices. Therefore, an improved MTJ stack is needed to enable a magnetic layer with thermal stability to at least 400° C., and where PMA is maintained in the reference layer and free layer.